The present invention relates to an exposure time control circuit for a camera having two automatic exposure time control functions (hereinafter referred to merely as "AUTO" when applicable) as well as a manual exposure time control (hereinafter referred to merely as "MANU" when applicable).
Cameras having two control functions according to the following systems have been proposed in the art.
(1) A first system in which "AUTO" is electrically implemented but "MANU" is mechanically implemented.
(2) A second system in which both "AUTO" and "MANU" are electrically implemented. In such a system, the time determination of one or both of "AUTO" and "MANU" is effected by a digital circuit.
(3) A third system in which both "AUTO" and "MANU" are electrically implemented. In this system, the resistance value of a resistor provided for "MANU" is varied by a so-called shutter dial.
However, a camera according to the first system is disadvantageous in that, since the manual exposure time control is mechanically implemented, the mechanism is rather intricate and the mechanism requires a rather large part of the space available in the camera. A camera according to the second system is also disadvantageous in that, since the time determination must be done with a digital circuit, the required electrical circuit is large in size and intricate and costly to manufacture thereby significantly contributing to increasing the total manufacturing cost. A camera according to the third system suffers from problems in that, when the space occupied by the resistor and the reliability of the connection between the resistor and its contactor are taken into account, the camera itself is low in reliability and moreover is difficult to miniaturize. In addition, a common drawback accompanying all three systems is that a shutter dial is required for manual exposure time control which is an obstruction to the effective utilization of the space available in the camera.
In the case where the background is much brighter than an object as in the case of taking a picture of a person on a slope or in the case where, contrary to the above-described case, an object is much brighter than the background as in the case of taking a picture of a person spotlighted, it is necessary for the operator of a camera which has the two functions described above to intentionally set the exposure factor to a value such as two, four times, 1/2 or 1/4 times what would otherwise be used. For this purpose, in the case of "AUTO", a technique has been proposed in which the resistance of a film sensitivity changing resistor is changed by a member such as a dial. In the case of "MANU" operation, a mechanical control technique utilizing a cam and a technique including changing the resistance of a manual exposure time setting resistor have been employed. In both of these cases, it is necessary to set a manual exposure time with a so-called "shutter dial".
There has been a strong demand for reducing the size and manufacturing cost of a camera. A camera having the two functions of "AUTO" and "MANU" is disadvantageous in this regard in that is is necessary to provide two members, an exposure factor setting member such as a dial and a shutter dial which are quite large in size and high in manufacturing cost. In addition, the camera surffers from a problem in that it requires a number of contact points between the mechanism and the resistors and accordingly its reliability is relatively low.